Prior ideas for generating control forces for stabilizing semi-submersible platforms in variable sea states involve tanks open at the bottom, located at the base of the columns which support the deck of the platform above the water surface. Such tanks may also be installed within the pontoons with open access at the bottom. Air pressure is then supplied to the enclosed space at the top of the tank in an amount necessary to balance the water pressure at the submerged open base, and varied in time about this average value to provide control forces on the platform necessary to suppress its motion in the water by using several such tanks in columns generally located at each corner of the platform. The prior art thus proposes to control roll and pitch motions in addition to the platform's response to heave. See, for example, U.S. Pat. Nos. 3,318,275 and 3,349,740.
It should be noted that in previous concepts such as described above, the effect of wave forces acting on the platform is unchanged by incorporation of such tanks in the columns or pontoons. Since the base areas of the columns or pontoons exposed to the static and dynamic sea forces would not be altered, the passive response characteristics of the platform would be unchanged whether or not the dynamic motion suppression system was in operation.
The inherent disadvantage of such prior art control systems is that the same equipment which provides the pressure necessary to develop the control forces also develops the pressure necessary to meet the static water pressure at the bottom of each of the columns. Moreover, the air pressure in the enclosed space at the top of the tanks is necessarily large to meet that static sea pressure as well as to provide additional increments of pressure for control forces when required by the dynamic motion suppression control system. The blowers necessary to provide the high ambient pressure and the large pressure variations due to waves are costly. In addition, to meet the peak flow and pressure required, large accumulators were necessary to store air at pressures of up to 60 PSIG. The volume of these accumulators was required to be on the order of 10,000 to 20,000 cubic feet. Such energy storage may be regarded as constituting a hazard to the over-all safety of the platform as well as adding to cost and space requirements. Finally, to install prior art control tanks, much internal modification and rearrangement of equipment of the platform is required.
Presently, ships or vessels having a vertical duct extending from the deck through the bottom of the hull and open to sea are used for offshore drilling and servicing operations. Such a duct is commonly referred to as a "moon pool", and constitutes a vertical water column. Derricks or cranes on the deck are used to lower equipment through such a duct to the sea floor. At other times, drilling operations are conducted through this duct.
The water level in the vertical duct rises and falls in response to periodically varying pressure at the base of the duct, the variation of pressure being induced by wave action. There is a frequency proportional to the effective length of the column at which the oscillating water column is naturally resonant, and at which small wave pressure changes produce greatly amplified oscillations of the water level in the duct. When equipment, such as a diving bell, is lowered through the duct on the way to emplacement on the sea floor, any oscillations in the level of the free surface of the column of the water can exert forces on the equipment being lowered. Such forces, for example, can momentarily raise the equipment, producing slack in the support cable, and then immediately thereafter cause the equipment to suddenly drop thus producing an impact stress on the cable, often causing the cable to break. Therefore, suppression of the oscillations of the "moon pool" water column is necessary to preclude the oscillatory forces caused thereby from endangering the crewman and integrity of the equipment and cable.